pg-dump-anon: use latest postgresql available

While reviewing #352966 I noticed that the pg_anonymizer test fails for
postgresql 17. The reason for that is that `pkgs.postgresql` is v16 and
using its psql to connect against a v17 database doesn't work.

I decided that we'll just use the latest available package in here. I
don't want to introduce another attribute (`postgresql_latest`), if
there are too many instances of that we're blocked on adding new
postgresql majors directly to master again which is the current status
quo. With the test rework in #352966 it's also way easier to catch this.

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